1. Field of the Invention
The present invention relates to a rolling-contact cylindrical element such as a tappet roller for use as a driven part such as a fuel injector, an intake or exhaust valve, or the like in an internal combustion engine.
2. Description of the Related Art
Conventionally, iron-based alloys such as a bearing steel and a tool steel, ceramics such as silicon nitride, and the like have been used as a rolling-contact cylindrical element such as a tappet roller. However,for example, there is the following problem. Because of the difference of parallelism between a cam and a contact surface, the deflection of the shaft of a fuel injection apparatus and intake and exhaust valves, there occurs partial contact of the cam with an end portion of the tappet roller and bearing pressure between the tappet roller and the cam increases. As a result, there occurs unstable rotation and the seizure, wear, pitching, and exfoliation of the contact surface.
Accordingly, a technique has been proposed to provide the contact surface of the tappet roller with crowning so as to alleviate the aforementioned partial contact due to a difference in parallelism and the deflection of a shaft, thereby suppressing the rise in bearing pressure and obtaining stable rotation. The shape of the bus of the crowning includes a so-called full crowning, a trapezoidal crowning, and a crowning combining circular arcs. For example, JP-U-5-36005 proposed that in order to overcome the problems of the trapezoidal crowning and the full crowning, a circular arc crowning having a large radius of curvature is provided in a central portion of the contact surface, while a circular arc crowning having a small radius of curvature is provided at an end portion thereof.
However, with tappet rollers in recent years, the bearing pressure with the cam is made high so as to effect control with higher accuracy by transmitting the rotation of the cam accurately to the fuel injector or the like. In addition, since fuel injection is effected under higher pressure as a countermeasure for controlling exhaust gases, the surface of contact with the cam has come to be subjected to extremely high contact pressure. For this reason, with the aforementioned crownings, even if they are able to transmit the rotation, excessively large bearing pressure occurs at the end portion, i.e., edge load occurs, due to misalignment during assembly, i.e., an inclination. Consequently, there are problems in that wear is promoted, variations occur in the transmission of rotation, the performance of the internal combustion engine declines, and the rolling life of the tappet roller decreases due to the exfoliation of the contact surface.
FIG. 9 shows a circular arc crowning of a general tappet roller. In this example, a half Le of the effective contact length is 8 mm, a circular arc crowning with a radius of curvature of 2, 000 mm is provided in its central portion, a circular arc crowning with a radius of curvature of 800 mm is provided in a 3-mm section ranging from an axial position, x=5 mm, with a center in the direction of the bus set as an origin to the end in such a manner as to continue therefrom. FIG. 10 shows the distribution of bearing pressure when the tappet roller of this crowning shape and the cam are brought into contact with each other with an inclination of 5/1,000. The abscissa in the drawing shows the ratio of the aforementioned axial position x with the center in the direction of the bus set as the origin to the half L.sub.e of the effective contact length, while the ordinate shows the ratio of bearing pressure P to Hertz's maximum bearing pressure P.sub.H. However, in this case, it is assumed that the Hertz's maximum bearing pressure is defined as a maximum contact bearing pressure occurring in the center of the contact surface when circular cylinders are brought into contact with each other under ideal conditions in which there is no misalignment and no edge load. Then, as is apparent from the drawing, an extreme edge load occurs in the conventional case.